All the production of primary aluminum metal in the world has been and is by the electrolytic dissociation of alumina dissolved in molten bath of cryolite using the Hall-Heroult process. Although the process has become many fold energy efficient and productive since its first commercialization in 1886, the basic process fundamentals have remained unchanged.
Alumina (aluminum oxide) is extracted from alumina bearing ore, bauxite, by a wet chemical process (Bayer process) using caustic soda under high temperature and pressures. Bauxite ore is a complex mixture of aluminum oxide, iron oxide and silicon dioxide (silica) and aluminum refining process dissolves alumina in caustic solution leaving most of the other impurities such as iron oxide, silica and many other oxides of trace elements outside of the solution which is filtered out and discarded as a waste, solution called red mud” (color red derived from the iron oxides). The alumina dissolved in the caustic solution is then precipitated out and calcined to produce metal grade alumina which meets strict composition specification for impurities especially. The impurity control of metal grade alumina is important since virtually all the impurities contained in alumina will end up in aluminum produced in the subsequent aluminum smelting process. It is economically and operationally difficult to produce alumina with varying degree of controlled impurities.
Alumina (Al203) is dissolved in molten cryolite (Na3AlF6) and is reduced to aluminum metal by direct current electrolysis in Hall Heroult aluminum smelting cell. The released oxygen rises through the electrolyte and reacts with the sacrificial carbon of the anode, while the molten aluminum settles to the carbon cathode bottom, of the reduction cell. The molten aluminum is periodically removed from the cell bottom by siphoning techniques.
The direct current enters the aluminum smelting cell from sacrificial solid carbon anodes. Burned carbon anodes (called anode butts) are periodically removed from the cell and are replaced by new anodes. Carbon anodes are made from mixing calcined petroleum coke and coal tar pitch, shaping them into “green anodes” which are thermally treated into finished baked anodes to drive out the hydrocarbon volatiles. Most of the calcined petroleum cokes are produced from calcined green coke obtained from the refining of crude oil into carbon residuals followed by extracting green coke in an apparatus called delayed coker. Most of the metallic impurities especially iron, silicon, nickel, vanadium from crude oil are directly transferred to calcined petroleum coke. In particular, the nickel and vanadium contents of calcined petroleum coke keeps increasing due to supply situation from the crude oil. Normally when such anodes are used in smelting cells, most of the nickel and vanadium contents are passed to the aluminum metal produced in a smelting cell. This is not desirable as the higher Ni and V contents produce aluminum metal with much higher nickel and vanadium contents. As it is practiced today, most of the metal impurities from petroleum coke are transferred to the finished aluminum metal.
FIG. 1 shows that vanadium has increased to 270 ppm on average in 2007. The next generation of green cokes products can go as high as 1000 ppm in vanadium content. A similar or worse trend for nickel is projected.
The impurities in both the alumina and carbon will remain in either the aluminum, the cryolite or evaporate with the generated CO2 and mixed gas and particulates from the electrolytic cell. For example CO2 and some of the heavy metal impurities evaporate in the form of relatively volatile fluoride salts. However, the majority of the heavy metallic impurities such as iron, silicon, nickel, and some of the vanadium, etc., precipitate with the primary aluminum and become an impurity in the aluminum.
Due to environmental concerns over fluoride emissions, the exhaust gases from the electrolytic cells are passed through the alumina (primary alumina). This primary alumina traps the majority of heavy metal fluorides and hydrogen fluoride escaping the cell. This dry scrubber alumina called secondary alumina is the primary source of alumina being fed to the electrolytic cell. Consequently, all the heavy metals in the alumina and carbon anodes eventually become part of the primary aluminum.
Tightened regulations on emissions, particularly those of fluoride species, are leading to the development of more efficient systems for cell gas collection and cleaning. With good pot hood collection efficiency, and the dry scrubber recovering and recycling most of the particulate and gaseous fluorides, the operation can be regarded as a closed loop between the cells and the scrubber. As the high particulate collection efficiency also includes those impurities lost to the duct, the recycling of the dry scrubber alumina (secondary or reacted alumina) naturally leads to increased impurity levels in the bath, and thus also in the metal.
Impurity elements unavoidably enter the process stream, mainly with the raw materials. Alumina, anode carbon and the bath material (cryolite and AIF3) are the predominant sources. The impurity levels found in the raw materials vary markedly. Furthermore, operational practices may also contribute on a significant level to the impurity burden, especially for iron and silicon. Although, the impurities can follow several different pathways; the main exit route for several key elements is with the metal. The mechanisms effecting impurity transport in reductions cells are not completely understood, but it is commonly agreed that the transport goes via the electrolyte. Thus, the total impurity burden becomes more important than the particular source of the impurity.
The impurities can be classified based on their behavior in the electrolytic process, including metallic species with higher reduction potential than Al203, metallic species with lower reduction potential than Al203, water and sulphur, and non-metallic oxides.
The elements in the metallic species with lower reduction potential than Al203 are of prime interest in the light of aluminum purity considerations. These elements exhibit different volatility in the electrolyte bath, and can therefore be classified further, such as non-volatile or low volatility impurities, partially volatile impurities and volatile impurities.